This invention relates to an optical fiber used in optical transmission such as wavelength division multiplexing (WDM) transmission, etc. in a wavelength band of e.g., 1.5 xcexcm, etc., and optical communication systems using this optical fiber.
A communication information capacity tends to be greatly increased as information society is developed. Techniques of the wavelength division multiplexing transmission (WDM transmission) and time division multiplexing (TDM) transmission are noticed as such information is increased. This wavelength division multiplexing transmission uses a system for transmitting signals of plural wavelengths by one optical fiber. Therefore, this system is an optical transmitting system suitable for high capacity and high bit-rate transmission. The wavelength division multiplexing transmission technique is vigorously studied at present.
It is considered at present that the wavelength division multiplexing transmission is performed in a wavelength band of 1.55 xcexcm as a gain band of an erbium-doped optical fiber amplifier. The wavelength band of 1.55 xcexcm is a wavelength band with 1550 nm in wavelength approximately as a center, e.g., as in a wavelength band from 1530 nm to 1570 nm.
However, there are problems of an increase in power of an optical signal and a non-linear phenomenon due to an interaction between signals, etc. to perform the wavelength division multiplexing transmission. Therefore, for example, it is reported in a society report document OFC"" 97 TuNlb of Japan, etc. to consider that a non-linear refractive index difference (n2) is reduced and restrained to restrain the non-linear phenomenon.
It is also noticed to consider that an effective core area (Aeff) of the optical fiber is increased together with this reduction in the non-linear refractive index difference. Distortion xcfx86NL of a signal due to the non-linear phenomenon is generally represented by the following formula (1). Therefore, when the effective core area of the optical fiber is increased, the waveform distortion of a signal due to the non-linear phenomenon can be reduced.
xcfx86NL=(2xcfx80xc3x97n2xc3x97Leffxc3x97P)/(xcexxc3x97Aeff)xe2x80x83xe2x80x83(1) 
In the formula (1), xcfx80, n2, Leff, P and xcex respectively designate a ratio of the circumference of a circle to its diameter, a non-linear refractive index, an effective optical fiber length, signal power and a signal optical wavelength.
Accordingly, it is very important to enlarge the effective core area in the optical-fiber used for e.g. the wavelength multiplexing transmission, and this enlargement is very noticed as reported in society report documents OFC"" 96 WK15 and OFC"" 97 YuN2 of Japan.
The present invention provides an optical fiber and an optical communication system using this optical fiber.
The optical fiber of the invention comprises:
an effective core area from 40 xcexcm2 to 60 xcexcm2 in a set wavelength band of at least one portion of a wavelength band of 1.5 xcexcm;
a dispersion value set to 4 ps/nm/km or more and 10 ps/nm/km or less at a wavelength of 1.55 xcexcm;
a dispersion slope set to a positive value equal to or smaller than 0.04 ps/nm2/km in a wavelength band of 1.55 xcexcm; and
a zero dispersion wavelength equal to or smaller than 1.4 xcexcm.